1. Field of the Invention
The present invention relates to air-fuel ratio control apparatus of internal combustion engine.
2. Related Background Art
For purifying exhaust gas from the internal combustion engine, an exhaust purifying catalyst (three-way catalyst) is placed on the exhaust path, an air-fuel ratio sensor is located on the exhaust path to detect the air-fuel ratio (A/F), and feedback control is implemented so as to control an air-fuel mixture toward the stoichiometric air-fuel ratio, thereby simultaneously decreasing nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC). The purification efficiency of exhaust gas emitted from the internal combustion engine is further increased effectively by carrying out the foregoing feedback control with accuracy. It is also effective to enhance the purification efficiency of nitrogen oxides NOx, carbon monoxide CO, and hydrocarbons HC by the oxygen occlusion function of the exhaust purifying catalyst.
Research has been conducted heretofore on control for making effective use of this oxygen occlusion function. A control apparatus noting the oxygen occlusion function is, for example, one described in Japanese Patent Application Laid-Open No. H05-195842. The control apparatus described in No. H05-195842 is configured to determine one target value as an amount of oxygen (oxygen storage amount) to be stored in the exhaust purifying catalyst and always control air-fuel ratios so that the oxygen storage amount becomes this target value.
The exhaust purifying catalyst possesses such a property that, when the catalyst is subjected to repetitions of oxidation/reduction reactions and oxygen adsorption/desorption reactions to some extent, catalyst metal is activated more and the purification performance (oxidation/reduction performance, oxygen adsorption/desorption performance) is maintained higher than otherwise. If the control to converge the oxygen storage amount on one target value is always kept on, there will occur a phenomenon in which the activation of catalyst metal is hindered, so as to degrade the exhaust purification performance. There were thus desires for improvement to overcome this problem.
Accordingly, an object of the present invention is to provide air-fuel ratio control apparatus of internal combustion engine that can suppress the degradation of the oxygen occlusion capability of the exhaust purifying catalyst and that can perform stable control of air-fuel ratio.
An air-fuel ratio control apparatus of an internal combustion engine according to the present invention comprises oxygen storage amount estimating means for estimating an oxygen storage amount of an exhaust purifying catalyst located on an exhaust path of the internal combustion engine, based on a history of an oxygen adsorption/desorption amount calculated from an air-fuel ratio of the internal combustion engine, and air-fuel ratio control means for controlling the air-fuel ratio, based on the oxygen storage amount estimated by the oxygen storage amount estimating means. An upper threshold and a lower threshold are set for the oxygen storage amount estimated by the oxygen storage amount estimating means. When the oxygen storage amount estimated by the oxygen storage amount estimating means is between the upper threshold and the lower threshold, the air-fuel ratio control means controls the air-fuel ratio to a constant value. When the oxygen storage amount estimated is larger than the upper threshold, the air-fuel ratio control means controls the air-fuel ratio to a rich region. When the oxygen storage amount estimated is smaller than the lower threshold, the control means controls the air-fuel ratio to a lean region. Therefore, according to the present invention, the oxygen storage amount varies between the upper threshold and the lower threshold (within a predetermined range) and it is thus feasible to promote the activation of the exhaust purifying catalyst, so as to keep the purification performance of the exhaust purifying catalyst high and to implement stable control of air-fuel ratio.
In a preferable aspect of the invention, a target storage amount is further set between the upper threshold and the lower threshold, and the air-fuel ratio control means performs such control that, when the oxygen storage amount estimated by the oxygen storage amount estimating means is larger than the upper threshold, the air-fuel ratio control means controls the air-fuel ratio to the rich region so that the oxygen storage amount becomes the target storage amount and that, when the oxygen storage amount estimated is smaller than the lower threshold, the air-fuel ratio control means controls the air-fuel ratio to the lean region so that the oxygen storage amount becomes the target storage amount. This makes it feasible to implement both keeping the oxygen storage amount within the predetermined range and also effecting the variation thereof.
In a further preferable aspect of the invention, two target storage amounts are set as an upper target storage amount and a lower target storage amount between the upper threshold and the lower threshold, and the air-fuel ratio control means performs such control that, when the oxygen storage amount estimated by the oxygen storage amount estimating means is larger than the upper threshold, the air-fuel ratio control means controls the air-fuel ratio to the rich region so that the oxygen storage amount becomes the upper target storage amount and that, when the oxygen storage amount estimated is smaller than the lower threshold, the air-fuel ratio control means controls the air-fuel ratio to the lean region so that the oxygen storage amount becomes the lower target storage amount. This further decreases chances of forcibly controlling the oxygen storage amount by the air-fuel ratio control based on the oxygen storage amount and can promote the activation of the exhaust purifying catalyst further more.